Coinhibitory signals attenuate T cell receptor signaling and inhibit T cell responses. PD-1 is the prototype coinhibitory receptor and has been shown to be highly expressed on exhausted hepatitis C virus (HCV)- specific T cells that have lost effector functions. Blockade of PD-1 with monoclonal antibodies (mAb) enhances HCV-specific CDS T cells responses in vitro. Our hypothesis is that coinhibitory pathways contribute to exhaustion of HCV-specific T cells, regulating progression to chronic infection, and that blockade of coinhibitory pathways will enhance effective anti-HCV immune responses. Recent work shows that additional coinhibitory pathways contribute to T cell exhaustion and that blockade of multiple coinhibitory pathways optimally enhances T cell responses. Core C will focus on the PD-1, CD160, LAG-3, CTLA4, and CD161 coinhibitory pathways identified by this work. Core C will generate and produce mAbs that will facilitate analysis of the function and expression of the PD-1/PD-1 Ligand pathway as well as other coinhibitory pathways including CD160, LAG-3, CTLA4, and CD161 pathways. Core C will generate novel dimeric and multimeric Ig fusion proteins of these coinhibitory pathway proteins in order to either block or transduce signals via cross-linking receptors. The capacity of these blocking mAbs to enhance HCV-specific T cell responses will be tested. These mAbs and Ig fusion proteins will be used in high throughput assays by Technology Development Project 1 to identify small molecule antagonists of coinhibitory pathways. Core C provides a critical means by which the U19 will achieve its goals of understanding how to manipulate the coinhibitory signals provided by PD-1, CD160, LAG-3, CTLA4, CD161 and their ligands as well as the mechanism of these inhibitory signals. Core C will work closely with project investigators, providing them mAbs and Ig fusion proteins as needed.